Cut cauliflower into florets (about 3 cups). Cook in boiling salted water to cover 10 to 12 minutes or until tender; drain and set aside.,Melt 4 tablespoons butter in a medium saucepan over medium-high heat. Whisk in flour until smooth. Gradually add milk, whisking constantly, until mixture begins to thicken. Add cheese and sour cream, stirring until cheese melts. (Do not boil.),______,Bake casserole, uncovered, at 350° for 45 minutes.